DNG is intended for raw files.
I don't think one can be dogmatic nowadays about DNG being intended for raw files. Everyone did understand DNG was just for raw files, but that's not been a simple distinction for some time. - you've been able to store JPEGs as DNGs, for example. Adobe are now making it even less clear with lossy DNG formats, and you've already got the beginnings of parametric ways to handle HDR and stitching.
Jusy curious, how about the reverse? Would there be any advantage you could think of to converting TIF to DNG when bringing them into LR?
@Steve House On balance, some. By saving those old original files as DNGs, you're making it much more obvious to yourself that these files are originals, while you might keep TIF for worked files. I find it handy to save scanned film images as DNGs and use it for the few JPEG originals I shoot. I instantly recognize DNGs as originals, and can and filter on them in Lightroom, for example. Because these scans or JPEGs are now DNG, there's no chance I'll overwrite the original image data in Photoshop.